1. Field of the Invention
The present invention relates to a deformable mirror device and an apparatus for observing retina of eye.
2. Related Art
In general, an apparatus for observing retina of eye is an apparatus which irradiates a retina of an eye to be examined with illumination light and receive and detect light of an image of the retina to be examined through an retina image forming optical system by using an image pickup device (for example, a CCD camera) in order to observe the retina of the eye. For detecting and preventing a disease concerning eyes, it is desirable that the detection accuracy and resolution are high as far as possible. Since an eyeball is not an ideal lens having no aberration, however, the eyeball has a wavefront aberration which becomes a factor of lowering the detection accuracy and resolution.
Therefore, a deformable mirror which can vary its surface shape on the basis of information supplied from a controller is provided between the image pickup device which detects the image of the retina and the retina of the eye to be examined. The image of the retina of the eye to be examined reflected by the deformable mirror is led to a wavefront sensor (for example, the Shack Hartmann sensor) to detect wavefront aberration. A control device indicates a displacement quantity to the deformable mirror so as to reduce or eliminate the wavefront aberration on the basis of the detected wavefront aberration. Owing to this indication, the shape of the deformable mirror is deformed and an image having no wavefront aberration is obtained by the image pickup device.
A deformable mirror having a shape variable by electrostatic sucking force is known (see, for example, FIG. 2 in JP-A 02-101402 (KOKAI)). The deformable mirror shown in FIG. 2 in JP-A 02-101402 (KOKAI) has a configuration obtained by forming a fixed electrode film 12 on an insulative substrate 11, a spacer part 18 having an opening in the center on the fixed electrode film 12, stacking a reflection film 17, a movable electrode film 16 and a SiO2 insulation film 14 on the spacer part 18 so as to cover the opening, and forming a silicon substrate 13 having an opening in the center on the stacked film. Therefore, the stacked film consisting of the reflection film 17, the movable electrode film 16 and the SiO2 insulation film 14 is disposed as a membrane part having a peripheral part fixed by the spacer part 18 and the silicon substrate 13 and a central part deformable by electrostatic force between the fixed electrode 12 and the movable electrode film 16.
In the membrane part, it is desirable that “generated force (load)—deflection characteristics” obtained when a predetermined voltage is applied between the fixed electrode and the movable electrode are uniform and symmetric in the plane as far as possible and their variations in manufacturing of the deformable mirror are small. This is because if there are variations it is necessary to execute adjustment work according to individual characteristics.
If the “generated force (load)—deflection characteristics” have a property that the membrane part tends to harden, then a greater voltage is needed to generate the same displacement. This can be a cause of hampering the fealizability and usefulness of a drive circuit and a peripheral circuit of the deformable mirror.
The “generated force (load)—deflection characteristics” are influenced by various causes. In the deformable mirror, however, residual stress remaining in a thin film or the like caused by difference in coefficient of linear thermal expansion between substances used as materials exerts a great influence. Furthermore, the “generated force (load)—deflection characteristics” depend on material characteristics (such as the Young's modulus and Poisson's ratio) of members used in the membrane part, flexural rigidity determined by characteristics (second moment of area) concerning the shape, and boundary conditions in regions for fixing or supporting peripheries of the membrane part. Residual stress remaining in the thin film or the like exerts an influence upon the above-described boundary conditions.
In the deformable mirror described in JP-A 02-101402 (KOKAI), except for the reflection film, the membrane part is a stacked film consisting of the movable electrode film 16 and the SiO2 insulation film 14, and the membrane part has a configuration fixed in its peripheries by the spacer part 18 and the silicon substrate 13. Therefore, the “generated force (load)—deflection characteristics” in the membrane part vary and have anisotropy (nonuniformity) in the plane, under the influence of the deflection in the region fixing the membrane part exerting upon the boundary conditions at a fixed point and the influence of the residual stress existing within the movable electrode film 16 and the insulation film 14 laminated in the membrane part. The above-described deflection is generated by the difference in coefficient of linear thermal expansion between the constituent films. As for the residual stress, the magnitude and the direction (tension/compression) of the residual stress are influenced by the process kind and procedure at the time of film forming.
The degree of influence of the residual stress and the boundary conditions upon the “generated force (load)—deflection characteristics” is very large. A small difference in the residual stress or the boundary conditions causes a great difference in an output result (the deflection quantity of the membrane). The design work concerning the structure of the membrane part and the arrangement and shape of electrodes provided on the substrate surface is complicated.